The present invention relates to a procedure and apparatus for the acquisition, processing, and inversion of two or more sets of data signals obtained from the same subsurface areaxe2x80x94preferably, but not restricted toxe2x80x94seismic data signals. In particular, the inversion method aims at estimating the subsidence and compaction of geological strata in the underground. Compaction can be furthermore inverted into material attributes such as acoustic impedance or strain and stress fields. These attributes have important relevance to avoid hazards when drilling new wells into the subsurface and for monitoring hydrocarbon reservoirs under production. This patent application is related to commonly-assigned International Patent Application PCT/IB99/01144 entitled xe2x80x9cMethod for Processing Time Lapsed Seismic Data Signalsxe2x80x9d, published Dec. 29, 1999 as WO99/67660, incorporated herein by reference.
Seismic data signals are typically acquired by measuring and recording data during a xe2x80x9c3D seismic surveyxe2x80x9d. A xe2x80x9c3D seismic surveyxe2x80x9d in general is performed by conducting a plurality of xe2x80x9cseismic experimentsxe2x80x9d i.e. typically by firing an impulsive seismic energy source at the surface of the earth/sea or seafloor and recording the received signals at a set of geo/hydro-phones. The geo/hydro-phones are typically situated at the same surface as the source, but laterally displaced on regular grid positions. However, there are situations where a non-regular distribution of the geo/hydro-phones is preferred and/or where the source and the set of geo/hydro-phones are positioned at different depth levels.
In a xe2x80x9c3D seismic surveyxe2x80x9d, one will typically displace the source and sets of geo-/hydro-phones at fixed intervals (e.g. 25 meters) and in a certain direction (the xe2x80x9cInlinexe2x80x9d direction) and repeat the seismic experiment of firing the source and recording the received signals. After completion of such an inline recording, one will repeat this procedure so the source and the set of receivers are displaced a certain distance perpendicular to the inline direction. By this, one will scan the surface of the earth over an area of interest and thus complete a 3D seismic survey. The recording of a single inline can also be denoted as a 2D seismic survey.
During a seismic experiment, when firing the seismic source, a pressure wave will be excited and propagate into the subsurface. The pressure wave reflects off interfaces between various earth layers (such as rock, sand, shale, and chalk layers), and propagates upwardly to the set of geo/hydro-phones, where respectively the particle velocity of the wave vibrations or the pressure oscillations of the wave are measured and recorded. The strength of the reflected wave is proportional to the amount of change in elastic parameters (represented e.g. through density, pressure velocity, and shear velocity) at the respective interfaces. Consequently, the data recorded by the set of geo/hydro-phones represents the elastic characteristics of the subsurface below the set of geo/hydro-phones. However, in order to arrive at volumetric images of the subsurface the recorded signals have to be processed using a (preferably state of the art) processing scheme. Essentially, such a scheme reduces noise and focuses and maps the seismic signals to the points where the reflections occurred.
Often two or more 3D seismic surveys are obtained from the same subsurface area but at different times, typically with time lapses of between a few month and a few years. In some cases, the seismic data signals will be acquired to monitor changes in the subsurface reservoirs caused by the production of hydrocarbons. The acquisition and processing of time-lapsed three dimensional seismic surveys over a particular subsurface area (commonly referred to in the industry as xe2x80x9c4Dxe2x80x9d seismic data) has emerged in recent years as an important new prospecting methodology.
The purpose of a 4D seismic survey is to monitor changes in the seismic data signals that can be related to detectable changes in geological parameters. These (not necessarily independent) geologic parameters include fluid fill, propagation velocities, porosity, density, pressure, temperature, settlement of the overburden, etc. Of primary interest are changes taking place in the hydrocarbon reservoir zones of the imaged subsurface volume. Analysing these changes together with petroleum production data assists the interpreter in understanding the complex fluid mechanics of the system of migration paths, traps, and draining or sealing faults making up the hydrocarbon reservoir. This provides information regarding how to proceed with the exploitation of the field: where to place new production wells to reach bypassed pay and where to place injectors for enhanced oil recovery. In the case of deciding where to place well trajectories, the situation in the reservoir overburden becomes of interest as well. It is important to know the in situ stress field and especially over-pressured zones to avoid well breakdowns. All this helps to produce a maximum quantity of hydrocarbons from the reservoir at a minimum of cost.
Two important 4D seismic attributes are subsidence and compaction/stretching (the rate of change in subsidence with depth). A conventional method to measure subsidence from seismic data is to interpret corresponding horizons on two surveys of a seismic time lapse data set and calculate the difference in the two-way traveltime (assuming that the depth coordinate of the subsurface volume is measured in time). Correspondingly, a measure for compaction is to estimate the subsidence for the upper and lower horizon delineating a geological layer and calculate the difference in subsidence.
It is an object of the present invention to provide an improved method of processing time-lapse seismic data signals to estimate subsidence and preferably compaction of the imaged subsurface volume. An advantage of the present invention is that it provides first a more robust/less noise affected compaction estimate and second an estimate with higher resolution in that there is preferably generated a compaction estimate for each volume element making up the subsurface volume instead of being restricted to layers defined by horizons.
Another important aspect of the present invention is a link demonstrating how to relate the kinematic effect of compaction to changes in elastic parameters such as acoustic impedance.
The present invention relates generally to the processing of time-lapsed data of a subsurface volume and more particularly to a method of estimating the subsidence and preferably the compaction or stretching of geological strata in the subsurface. Another aspect of the invention is how to refine a compaction estimate into an estimate indicating the relative change in acoustic impedance.
In one embodiment, the method involves collecting two time-lapsed sets of seismic data and generating a new data volume indicating the amounts and direction (upwards or downwards) by which the samples of the first seismic data set have to be translated in order to arrive at a representation that best resembles the second seismic data set. Subsequently the derivative with respect to the depth direction may be calculated to arrive at the compaction estimate. Recognising that compaction corresponds to an increase in density; an empirical mapping relates compaction to changes in the relative acoustic impedance.
The method is of benefit in the field of monitoring hydrocarbon reservoirs with time lapsed measurements and will give indications of undrained reservoir areas and possible stress regimes in the overburden. The invention and its benefits will be better understood with reference to the detailed description below and the accompanying drawings.